Document: COR-3-99-144

Effects of nutrient limitation, light limitation, and invertebrate grazers on phytoplankton living in deep chlorophyll layers.

LEW SAWATZKY, C.*, W.A.WURTSBAUGH and C.LUECKE

Utah State University, Logan, UT 84321 USA ¹

Abstract:
Our study addressed factors controlling deep chlorophyll maxima (DCM) that occur in unproductive lakes and oceans. These chlorophyll peaks represent communities of phytoplankton living in deep, poorly-lit strata below the surface mixed layer and can be responsible for greater than 50% of lake primary production. The importance of nutrient limitation, light limitation, and herbivore grazing/recycling for the formation and maintenance of DCM were studied in two Sawtooth Mountain lakes of Idaho using microcosm experiments and physiological nutrient deficiency indicators. For Yellow Belly Lake, the results of the microcosm experiments show nitrogen limitation early in the summer growing season for the epilimnetic strata. As the summer progressed, co-limitation by nitrogen and phosphorus developed for both the epilimnion and metalimnion. Zooplankton, present at extremely high biomass due to low fish densities, had negative impacts, decreasing growth rate as compared to controls by a factor of up to six. In Stanley Lake, microcosm results show spatial and temporal variation between N and P as limiting nutrients in the surface layer. Phosphorus inhibition was observed at depths of 9-12m. Zooplankton had both positive and negative effects on growth rate depending on density. Stanley Lake, with its large fish population, has much lower zooplankton densities resulting in lower grazing pressures for phytoplankton. The nutrient deficiency indicators documented the variability in nutrient limitation in both lakes as well. The microcosm experiments also demonstrated that phytoplankton living at low light intensity were adjusted to these light conditions because providing them with more light resulted in growth rates up to 36 times lower than controls.

Keywords: deep chlorophyll maxima

This abstract is being presented at: 11:30 AM in session:
Oral Session #37: Phytoplankton.